Method for producing a water-absorbent resin

ABSTRACT

A method for producing a water-absorbent resin comprising at least two stages of reversed-phase suspension polymerization, wherein the first stage of the at least two stages of reversed-phase suspension polymerization comprises the following steps of: (A) performing a primary dispersion by stirring to mix in the absence of surfactants an aqueous solution of a water-soluble ethylenically unsaturated monomer containing a hydrophilic polymeric dispersion agent with a petroleum hydrocarbon dispersion medium in which a hydrophobic polymeric dispersion agent dissolves or disperses; (B) further performing a secondary dispersion by adding a surfactant to the resultant dispersion liquid; and (C) performing a polymerization by using a water-soluble radical polymerization initiator to obtain a water-absorbent resin as particles in a hydrous gel state which disperse in the petroleum hydrocarbon dispersion medium, and a water-absorbent resin obtained by the method.

TECHNICAL FIELD

The present invention relates to a method for producing awater-absorbent resin, and to a water-absorbent resin obtained by thesame. More specifically, the present invention relates to a method forproducing a water-absorbent resin comprising at least two stages ofreversed-phase suspension polymerization, wherein an odor originatedfrom a raw material component, in particular, a petroleum hydrocarbondispersion medium, is further reduced as compared with a water-absorbentresin obtained by a conventional method, and to a water-absorbent resinobtained by the method.

BACKGROUND ART

Water-absorbent resins are widely used in hygienic materials such asdisposable diapers and sanitary napkins; daily commodities such as petsheets; water absorbing sheets for food products; industrial materialssuch as water blocking materials for cables; water retention agents forgreening/agriculture/horticulture; and the like.

Hygienic materials such as disposable diapers and sanitary napkins aregenerally constituted with a top sheet, a back sheet, a hot meltadhesive, an elastic material, a water-absorbent resin and a pulp fiber,various synthetic resins and modifiers are used. Therefore, an odororiginating from raw material components is perceived from the hygienicmaterials, in some cases. Since these hygienic materials are put on thehuman body, the odor makes users uncomfortable even if it is subtle and,therefore, it is desired to develop an odor-free material.

Among constituent materials of these hygienic materials, thewater-absorbent resin has a subtle odor originating from the substancesused in the production process, and since the odor tends to emit uponwater absorption, it is considered to be desirable to reduce the odor.

As water-absorbent resins used for hygienic materials, for example, apartially-neutralized product of polyacrylic acid, a neutralized productof a starch-acrylic acid graft polymer, a hydrolysate of astarch-acrylonitrile graft copolymer, a saponified product of a vinylacetate-acrylic acid ester copolymer are known.

As methods for producing such water-absorbent resins, an aqueouspolymerization method and a reversed-phase suspension polymerizationmethod are known. In a case where a water-absorbent resin is produced bya reversed-phase suspension polymerization method in whichpolymerization is performed by suspending a water-soluble monomer in adispersion medium, a major cause of the odor is considered to originatefrom the dispersion medium.

As conventional methods for producing the water-absorbent resin by areversed-phase suspension polymerization method, known are a method ofpolymerizing an aqueous solution of α,β-unsaturated carboxylic acid andalkali metal salt thereof in a petroleum hydrocarbon solvent using aradical polymerization initiator in the presence or absence of ainternal-crosslinking agent in which a sucrose fatty acid ester is usedas a protective colloid agent (see Patent Document 1), and a method ofpolymerizing a 25% by mass or more of aqueous solution of anα,β-unsaturated carboxylic acid and alkali metal salt thereof in apetroleum hydrocarbon solvent using a radical polymerization initiatorin the presence or absence of a internal-crosslinking agent in which apolyglyceryl fatty acid ester with an HLB of 2 to 16 is used as asurfactant (see Patent Document 2). However, these production methods donot focus on reduction of an odor, and thus odors of the resultantwater-absorbent resins are not sufficiently low.

Moreover, on a purpose for reducing an odor of a water-absorbent resin,the present inventors found that an odor originated from the dispersionmedium upon water absorption can be reduced by dispersing an aqueoussolution of a water-soluble ethylenically unsaturated monomer in thepetroleum hydrocarbon dispersion medium to which surfactants are notadded, and further adding a surfactant to the resultant dispersionliquid to further disperse and polymerize it (see Patent Document 3) ina reversed-phase suspension polymerization method; or by adding ansurfactant to a dispersion liquid obtained by dispersing an aqueoussolution of a water-soluble ethylenically unsaturated monomer in apetroleum hydrocarbon dispersion medium, in a first stage reversed-phasesuspension polymerization upon multi-stages of two or more stages ofreversed-phase suspension polymerizations (see Patent Document 4)

However, when a large amount of water-absorbent resins are used in thesemethods in the conventional method, an odor originated from thedispersion medium may be perceived upon water absorption, and therebythere is a need for further reducing the odor.

REFERENCE DOCUMENTS Patent Documents

-   [Patent Document 1] JP-A No. 61-87702-   [Patent Document 2] JP-A No. 62-172006-   [Patent Document 3] WO 2007/126002-   [Patent Document 4] WO 2009/025235

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention relates to a method for producing awater-absorbent resin, and a water-absorbent resin obtained by themethod. More specifically, an object of the present invention is toprovide a method for producing a water-absorbent resin comprising atleast two stages of reversed-phase suspension polymerization, wherein anodor originated from a raw material component, in particular, apetroleum hydrocarbon dispersion medium, is further reduced as comparedwith a water-absorbent resin obtained by a conventional method, and to awater-absorbent resin obtained by the method.

Means for Solving the Problems

The present inventors intensively studied about a relation between anodor originated from a petroleum hydrocarbon dispersion medium when thewater-absorbent resin absorbs water, and a petroleum hydrocarbondispersion medium used in production of the water-absorbent resin. As aresult of the study, the inventors have found out that in a method forproducing a water-absorbent resin comprising at least two stages ofreversed-phase suspension polymerization the odor is significantlyreduced as compared with that in a water-absorbent resin obtained by aconventional method by adding a hydrophilic polymeric dispersion agentin an aqueous solution of a water-soluble ethylenically unsaturatedmonomer containing a water-soluble radical polymerization initiator whenthe above-mentioned aqueous solution of monomer is primarily dispersedin a petroleum hydrocarbon dispersion medium in the absence ofsurfactants and in the presence of a hydrophobic polymeric dispersionagent in the first stage of the reversed-phase suspensionpolymerization.

That is, the present invention relates to a method for producing awater-absorbent resin shown below, and a water-absorbent resin obtainedby the method.

Item 1. A method for producing a water-absorbent resin comprising atleast two stages of reversed-phase suspension polymerization, whereinthe first stage of the at least two stages of reversed-phase suspensionpolymerization comprises the following steps of:

(A) performing a primary dispersion by stirring to mix in the absence ofsurfactants an aqueous solution of a water-soluble ethylenicallyunsaturated monomer containing a hydrophilic polymeric dispersion agentwith a petroleum hydrocarbon dispersion medium in which a hydrophobicpolymeric dispersion agent dissolves or disperses;

(B) further performing a secondary dispersion by adding a surfactant tothe resultant dispersion liquid; and

(C) performing a polymerization by using a water-soluble radicalpolymerization initiator to obtain a water-absorbent resin as particlesin a hydrous gel state which disperse in the petroleum hydrocarbondispersion medium.

Item 2. The method for producing a water-absorbent resin according toItem 1, further comprising: precipitating at least a part of thesurfactant in a slurry of the dispersion medium containing the particlesin a hydrous gel state obtained in the step (C); and

further performing two or three stages of reversed-phase suspensionpolymerization by adding an aqueous solution of the water-solubleethylenically unsaturated monomer.

Item 3. The method for producing a water-absorbent resin according toItem 1 or 2, wherein the hydrophilic polymeric dispersion agent is atleast one kind selected from the group consisting of polyvinylpyrrolidone and polyvinyl alcohol.

Item 4. The method for producing a water-absorbent resin according toany one of Items 1 to 3, wherein the hydrophobic polymeric dispersionagent is at least one kind selected from the group consisting of maleicanhydride-modified polyethylene, maleic anhydride-modifiedpolypropylene, maleic anhydride-modified ethylene-propylene copolymer,maleic anhydride-propylene copolymer, maleicanhydride-ethylene-propylene copolymer, polyethylene, polypropylene,ethylene-propylene copolymer, oxidized polyethylene, oxidizedpolypropylene, and an oxidized ethylene-propylene copolymer.

Item 5. The method for producing a water-absorbent resin according toany one of Items 1 to 4 wherein the surfactant is at least one kindselected from the group consisting of polyglyceryl fatty acid ester,sucrose fatty acid ester, and sorbitan fatty acid ester.

Item 6. The method for producing a water-absorbent resin according toany one of Items 1 to 5, wherein the water-soluble ethylenicallyunsaturated monomer is at least one kind selected from the groupconsisting of acrylic acid and its salt, methacrylic acid and its salt,and acrylamide.

Item 7. The method according to any one of Items 1 to 6, wherein thepetroleum hydrocarbon dispersion medium is at least one kind selectedfrom the group consisting of an aliphatic hydrocarbon and an alicyclichydrocarbon, having a carbon number of 6 to 8.

Item 8. A water-absorbent resin obtained by the method according to anyone of Items 1 to 7.

Effects of the Invention

According to the present invention, a method for producing awater-absorbent resin, wherein an odor originated from a raw materialcomponent, in particular, a petroleum hydrocarbon dispersion medium, isreduced, and a water-absorbent resin obtained by the method areprovided.

MODES FOR CARRYING OUT THE INVENTION

An origin of odor when a water-absorbent resin obtained by areversed-phase suspension polymerization absorbs water, is mainly adispersion medium remained within particles of the water-abosorbentresin. The present inventors have found out that a mechanism of adispersion medium remaining in water-absorbent resin particles is causedby generating so-called capsule-like water-absorbent resin particleswhich include the dispersion medium based on generating a liquid droplethaving a shape in which the disppersion medium which is an oil phase isentrapped in a liquid droplet of the aqueous monomer solution, namely,an O/W/O (oil/water/oil) type droplet, and then stabilizing andpolymerizing this O/W/O type droplet itself, upon dispersing the aqueousmonomer solution in the dispersion medium by means of stirring and thelike.

The method for producing a water-absorbent resin of the presentinvention is characterized in that a hydrophilic polymeric dispersionagent exists in an aqueous solution of a water-soluble ethylenicallyunsaturated monomer when the above-mentioned aqueous solution of monomercontaining a water-soluble radical polymerization initiator is primarilydispersing in a petroleum hydrocarbon dispersion medium in the absenceof surfactants and in the presence of a hydrophobic polymeric dispersionagent, upon performing a reversed-phase suspension polymerization, andthereby generation of O/W/O (oil/water/oil) type droplet is suppressedas compared with the conventional method, and polymerization isperformed by using the water-soluble radical polymerization initiatorafter performing a secondary dispersion by adding a surfactant. An O/W/Otype droplet is an abbreviation of (Oil in Water) in Oil, and refers toa state in which fine oil droplets are dispersed in water droplets, andthe water droplets are further dispersed in an oil phase. Namely, it isconstituted of an innermost oil phase, an intermediate water phase andan outermost oil phase. In the present invention, the O/W/O type dropletexhibits a state in which droplets of the aqueous solution of themonomer (water phase) contain smaller droplets of dispersion medium (oilphase).

More specifically, the production method of the present inventioncomprises at least two stages of reversed-phase suspensionpolymerization, wherein the first stage polymerization of the at leasttwo stages of reversed-phase suspension polymerization comprises thefollowing steps (A)-(C):

(A) performing a primary dispersion by stirring to mix in the absence ofsurfactants an aqueous solution of a water-soluble ethylenicallyunsaturated monomer containing a hydrophilic polymeric dispersion agentwith a petroleum hydrocarbon dispersion medium in which a hydrophobicpolymeric dispersion agent dissolves or disperses;

(B) further performing a secondary dispersion by adding a surfactant tothe resultant dispersion liquid; and

(C) performing a polymerization by using a water-soluble radicalpolymerization initiator to obtain a water-absorbent resin as particlesin a hydrous gel state which disperse in the petroleum hydrocarbondispersion medium.

According to the method comprising these steps, a water-absorbent resinwhich contains a reduced amount of a remaining petroleum hydrocarbondispersion medium can be obtained. In the present invention, the “amountof a remaining petroleum hydrocarbon dispersion medium” (Amount ofremaining dispersion medium) is a value measured by a measuring methoddescribed hereinafter.

The present invention comprises dispersing an aqueous solution of awater-soluble ethylenically unsaturated monomer in two steps of: a“primary dispersion” in which the aqueous solution of a water-solubleethylenically unsaturated monomer (hereinafter, “aqueous monomersolution” means the “aqueous solution of the water-soluble ethylenicallyunsaturated monomer” unless otherwise expressly indicated) containing ahydrophilic polymeric dispersion agent is mixed and dispersed in apetroleum hydrocarbon dispersion medium (hereinafter, “dispersionmedium” means the “petroleum hydrocarbon dispersion medium” unlessotherwise expressly indicated) in the absence of surfactants and in thepresence of hydrophilic and hydrophobic polymeric dispersion agents inthe step (A), and a “secondary dispersion” in which a surfactant isadded to intend a dispersion stabilization with the surfactant in thestep (B), and thereby performs a reversed-phase suspensionpolymerization. The present invention is intended to reduce an amount ofthe remaining dispersion medium by suppressing generation of O/W/O typedroplet as compared with conventional methods.

Examples of the water-soluble ethylenically unsaturated monomer used inthe step (A) include monomers having an acid group, such as(meth)acrylic acid [“(meth)acrylic” means “acrylic” and “methacrylic”,the same shall apply hereinafter],2-(meth)acrylamide-2-methylpropanesulfonic acid and maleic acid, andsalts thereof; nonionic unsaturated monomers such as (meth)acrylamide,N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate andN-methylol(meth)acrylamide; and amino group-containing unsaturatedmonomers such as diethylaminoethyl (meth)acrylate and diethylaminopropyl(meth)acrylate, and quaternized monomers thereof. These water-solubleethylenically unsaturated monomers may be used alone, or two or morekinds of them may be used in combination.

Among water-soluble ethylenically unsaturated monomers, (meth)acrylicacid and a salt thereof, and (meth)acrylamide are preferable from aviewpoint of industrial availability.

When the water-soluble ethylenically unsaturated monomer has an acidgroup, it can also be used as a salt after neutralizing the acid group.

Examples of an alkaline compound used when a monomer having an acidgroup is neutralized to a salt include compounds of lithium, sodium,potassium and ammonium. More specifically, examples of the alkalinecompound include sodium hydroxide, potassium hydroxide, lithiumhydroxide, sodium carbonate and ammonium carbonate.

When the monomer having an acid group is neutralized, a neutralizationdegree is preferably from 30 to 90 mol % of the acid group of thewater-soluble ethylenically unsaturated monomer. When the neutralizationdegree is less than 30 mol %, the acid group is not easily ionized andwater-absorption capacity deteriorates, and therefore it is notpreferred. When the neutralization degree is more than 90 mol %, safetyissues may arise when used as hygienic materials, and therefore it isnot preferred. For the timing of neutralization, it is common to performit in a monomeric state from a viewpoint of a degree of homogeneity.However, so-called post-neutralization by adding the above alkalinecompound to polymer after polymerization of monomers for neutralization,may be applied together.

In the present invention, a water-soluble ethylenically unsaturatedmonomer is used as an aqueous solution. The concentration of the monomerin the aqueous solution of a water-soluble ethylenically unsaturatedmonomer is preferably from 20% by mass to saturation concentration. Ifnecessary, the aqueous solution of a water-soluble ethylenicallyunsaturated monomer may contain a chain transfer agent and the like.

Examples of the chain transfer agent include compounds such as thiols,thiolic acids, secondary alcohols, hypophosphorous acid and phosphorousacid. These chain transfer agents may be used alone, or two or morekinds of them may be used in combination.

Examples of the petroleum hydrocarbon dispersion medium includealiphatic hydrocarbon having a carbon number of 6 to 8, such asn-hexane, n-heptane, 2-methylhexane, 3-methylhexane,2,3-dimethylpentane, 3-ethylpentane and n-octane; alicyclic hydrocarbonshaving a carbon number of 6 to 8, such as cyclohexane,methylcyclohexane, cyclopentane, methylcyclopentane,trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane andtrans-1,3-dimethylcyclopentane; and aromatic hydrocarbons such asbenzene, toluene and xylene. Among these hydrocarbon dispersion media,aliphatic hydrocarbons having a carbon number of 6 to 8, such asn-heptane, 2-methylhexane, 3-methylhexane and n-octane; and alicyclichydrocarbons having a carbon number of 6 to 8, such as cyclohexane,methylcyclopentane and methylcyclohexane are preferably used fromviewpoints of easy industrial availability and safety. These hydrocarbondispersion media may be used alone, or two or more kinds of them may beused in combination.

Further, among these hydrocarbon dispersion media, n-heptane andcyclohexane are preferably used from a viewpoint that a state of W/Otype reversed suspension is good, suitable particle size is easilyobtained, and that an industrial availability is easy and a quality isstable. Further, as an example of a mixture of the above-mentionedhydrocarbon, a commercially available Exxsol Heptane (manufactured byExxon Mobil Co.: containing heptane and isomeric hydrocarbons of 75 to85%) and the like may be used to obtain a suitable result.

The amount of the petroleum hydrocarbon dispersion medium to be used isusually from 50 to 600 parts by mass, more preferably from 50 to 400parts by mass, and still more preferably from 50 to 200 parts by mass,based on 100 parts by mass of the aqueous solution of a water-solubleethylenically unsaturated monomer from a viewpoint of uniformlydispersing of the aqueous solution of a water-soluble ethylenicallyunsaturated monomer and facilitating control of the polymerizationtemperature.

In the step (A), when the aqueous solution of a water-solubleethylenically unsaturated monomer is added and primarily dispersed inthe petroleum hydrocarbon dispersion medium in the absence ofsurfactants, the amount of the remaining petroleum hydrocarbondispersion medium can be reduced to a lower level by dispersing thewater-soluble ethylenically unsaturated monomer in the presence of ahydrophobic polymeric dispersion agent.

As the hydrophobic polymeric dispersion agent, it is preferred to selectand use those which are dissolved or dispersed in the petroleumhydrocarbon dispersion medium to be used, and examples of thehydrophobic polymeric dispersion agent include those having aviscosity-average molecular weight of 20,000 or less, preferably 10,000or less, and more preferably 5,000 or less. Specific examples thereofinclude maleic anhydride-modified polyethylene, maleicanhydride-modified polypropylene, a maleic anhydride-modifiedethylene-propylene copolymer, a maleic anhydride-ethylene copolymer, amaleic anhydride-propylene copolymer, a maleicanhydride-ethylene-propylene copolymer, polyethylene, polypropylene, anethylene-propylene copolymer, oxidized polyethylene, oxidizedpolypropylene, an oxidized ethylene-propylene copolymer, anethylene-acrylic acid copolymer, ethyl cellulose, ethylhydroxyethylcellulose, anhydrous maleinated polybutadiene and anhydrous maleinatedEPDM (ethylene/propylene/diene terpolymer).

Among them, at least one kind selected from the group consisting ofmaleic anhydride-modified polyethylene, maleic anhydride-modifiedpolypropylene, a maleic anhydride-modified ethylene-propylene copolymer,a maleic anhydride-ethylene copolymer, a maleic anhydride-propylenecopolymer, a maleic anhydride-ethylene-propylene copolymer,polyethylene, polypropylene, an ethylene-propylene copolymer, oxidizedpolyethylene, oxidized polypropylene and an oxidized ethylene-propylenecopolymer is preferred.

The amount of the hydrophobic polymeric dispersion agent to be added ispreferably 5 parts by mass or less, more preferably from 0.01 to 3 partsby mass, and still more preferably from 0.05 to 2 parts by mass, basedon 100 parts by mass of the aqueous solution of a water-solubleethylenically unsaturated monomer. When the amount of the hydrophobicpolymeric dispersion agent to be added is more than 5 parts by mass, itis not economic, being not preferable.

It is important that a hydrophobic polymeric dispersion agent is addedto a petroleum hydrocarbon dispersion medium, and then the abovedispersion medium is warmed once to establish a state where a part orwhole of the hydrophobic polymeric dispersion agent is dissolved ordispersed thereafter an aqueous monomer solution is added. There is noproblem even if the above dispersion medium is cooled after heating toperform an addition of the aqueous monomer solution in a state where apart or whole of the hydrophobic polymeric dispersion agent isprecipitated to be dispersed in cloud state.

When the aqueous solution of a water-soluble ethylenically unsaturatedmonomer is added to and dispersed in the petroleum hydrocarbondispersion medium, the aqueous solution of the water-solubleethylenically unsaturated monomer is dispersed by stirring. However,stirring conditions vary depending on a desired dispersed dropletdiameter and, therefore cannot be determined unconditionally.

The dispersed droplet diameter can be adjusted dispersed dropletdiameter can be adjusted by changing a type, size, rotation numbers of astirring impeller.

It is possible to use, as a stirring impeller, a propeller impeller, apaddle impeller, an anchor impeller, a turbine impeller, a Pfaudlerimpeller, a ribbon impeller, a FULLZONE impeller (manufactured by ShinkoPantech Co., Ltd.), a MAXBLEND impeller (manufactured by Sumitomo HeavyIndustries, Ltd.) and Super-Mix (manufactured by Satake ChemicalEquipment Mfg., Ltd.).

The present invention is characterized by not only the presence of ahydrophobic polymeric dispersion agent in a dispersion medium, but alsothe presence of a hydrophilic polymeric dispersion agent in an aqueousmonomer solution upon the primary dispersion. It is preferred that thehydrophilic polymeric dispersion agent is used in a state where it ispreviously added, dissolve to mix with an aqueous solution of thewater-soluble ethylenically unsaturated monomer. Generation of O/W/Otype particles can be suppressed at a lower level by primarilydispersing the aqueous monomer solution containing a hydrophilicpolymeric dispersion agent into a petroleum hydrocarbon dispersionmedium in the presence of the above-mentioned hydrophobic polymericdispersion agent and in the absence of surfactants.

As a hydrophilic polymeric dispersion agent, polyvinyl pyrrolidone(Abbreviated name “PVP”), polyvinyl alcohol (Abbreviated name “PVA”),polyglycerol, polyacrylate and the like can be used. These hydrophilicpolymeric dispersion agents may be used alone, or two or more of themmay be used in combination. Especially, polyvinyl pyrrolidone andpolyvinyl alcohol are preferred because they can be easily handled froma viewpoint of solubility in water and the like, and they easily exerttheir effect.

An amount of the hydrophilic polymeric dispersion agent can notdetermined unconditionally, because its preferred amount variesdepending on a kind and molecular weight of the hydrophilic polymericdispersion agent to be used. However, it is preferably 0.1-7 parts bymass, more preferably 0.3-5 parts by mass, and most preferably 0.5-3parts by mass based on 100 parts by mass of a water-soluble ethylenemonomer. When the amount of the hydrophilic polymeric dispersion agentto be used is less than 0.1 part by mass, the reduction effect for theremaining amount of the dispersion medium can not be sufficientlyacquired. When the amount of the hydrophilic polymeric dispersion agentto be used is more than 7.0 parts by mass, it is not preferred since theviscosity of an aqueous monomer solution increases, stirring rate isneeded to be greatly increased in order to obtain a targeted dropletdiameter, and thereby O/W/O type droplets are easily generated.

Although a molecular weight and the like of the hydrophilic polymericdispersion agent is not limited in particular, they are within a rangewhere the hydrophilic polymeric dispersion agent can be added todissolve in an aqueous monomer solution. For example, for the polyvinylpyrrolidone, the grade of K-15 to K-120 can be used, but K-30 to K-90are easily used from the viewpoint of the reduction effect of theremaining dispersion medium.

Moreover, although the degrees of saponification and the like ofpolyvinyl alcohol are not limited in particular, the degrees ofsaponification of not less than 85% is preferred. Further, Examplessuitably used as polyvinyl alcohol include those having a degree ofpolymerization of approximate 100 to 3,000, and those is easily usedfrom viewpoints of a reducing effect of the remaining dispersion mediumand use after dissolving.

Although polyvinyl pyrrolidone and polyvinyl alcohol are generally usedas a water-soluble thickener, the present invention utilizes effectsdifferent from those for so-called thickener of a water soluble polymer.According to the present inventors' study, for example cellulosederivatives, such as hydroxyethyl cellulose and ethyl cellulose, and thenatural polysaccharides of guar gum and glucomannan, and the like do nothave the reduction effect even by addition to a monomer aqueoussolution, the amount of the remaining dispersion medium tends toincrease. Thereby, it has been found that the generation of O/W/O typedroplets is not suppressed only by simply increasing the viscosity of anaqueous monomer solution.

Although a mechanism for reduction in the amount of the remainingdispersion medium by using a hydrophilic polymeric dispersion agent, isnot clear, it is speculated that with technologies of Patent Documents 3and 4, when the aqueous monomer solution is dispersed in a dispersionmedium in the absence of surfactants and in the presence of ahydrophobic polymeric dispersion agent, the hydrophobic polymericdispersion agent protects the surface of an aqueous monomer solutiondroplet in a dispersion-medium phase, and thereby the aqueous solutiondroplet is stabilized to inhibit that the dispersion agent is includedin the interior of the aqueous solution droplet by collisions ofdroplets. In contrast, it is speculated that when a hydrophilicpolymeric dispersion agent is further used as in the present invention,the hydrophilic polymeric dispersion agent can protect the surface of anaqueous solution droplet from the interior (water phase side) of themonomer aqueous solution droplet, and that the aqueous solution dropletis stabilized more than in the technologies of Patent documents 3 and 4,and thereby prevents not only inclusion but also dissolution into thedroplet of the dispersion medium.

A surfactant is added to a primary dispersion liquid obtained in thestep (A) and the aqueous solution of a water-soluble ethylenicallyunsaturated monomer is secondarily dispersed in the petroleumhydrocarbon dispersion medium (step (B)).

Examples of the surfactant used in the step (B) include nonionicsurfactants such as sorbitan fatty acid ester, polyoxyethylene sorbitanfatty acid ester, polyglyceryl fatty acid ester, polyoxyethyleneglyceryl fatty acid ester, sucrose fatty acid ester, sorbitol fatty acidester, polyoxyethylene sorbitol fatty acid ester, polyoxyethylene alkylether, polyoxyethylene alkyl phenyl ether, polyoxyethylene castor oil,polyoxyethylene hydrogenated castor oil, alkyl allyl formaldehydecondensed polyoxyethylene ether, polyoxyethylene polyoxypropyl alkylether, polyethylene glycol fatty acid ester, alkyl glucoside, N-alkylgluconamide, polyoxyethylene fatty acid amide and polyoxyethylenealkylamine; and anionic surfactants such as fatty acid salt,alkylbenzene sulfonate, alkylmethyl taurate, polyoxyethylene alkylphenyl ether sulfate, polyoxyethylene alkyl ether sulfate,polyoxyethylene alkyl ether sulfonic acid and a salt thereof,polyoxyethylene alkyl phenyl ether phosphoric acid and a salt thereof,and polyoxyethylene alkyl ether phosphoric acid and a salt thereof.These surfactants may be used alone, or two or more kinds of them may beused in combination.

Among these surfactants, at least one kind selected from the groupconsisting of polyglyceryl fatty acid ester, sucrose fatty acid esterand sorbitan fatty acid ester are preferred from a viewpoint ofdispersion stability of the aqueous solution of a water-solubleethylenically unsaturated monomer.

The amount of the surfactant to be added in the step (B) is preferablyfrom 0.01 to 5 parts by mass, and more preferably from 0.05 to 3 partsby mass, based on 100 parts by mass of the aqueous solution of awater-soluble ethylenically unsaturated monomer. When the amount of thesurfactant to be added is less than 0.01 part by mass, dispersionstability of the aqueous monomer solution deteriorates, and therefore itis not preferred. When the amount of the surfactant to be added is morethan 5 parts by mass, it is not economic, being not preferable.

A form of the surfactant added in the step (B) is not limited inparticular, a method which uses a surfactant previously diluted ordissolved in a small amount of the dispersion medium is preferredbecause the surfactant is dispersed and stabilized within a shortperiod. Besides, after adding a surfactant in the step (B), a stirringrotation number of a stirring impeller may be increased.

Based on the stirring rotation number after increasing the rotationnumber, a final dispersed droplet diameter is determined and theparticle size in the first stage of polymerization stage is determined.By setting the stirring rotation number for the primary dispersion inthe step (A) such that it is somewhat lower than the stirring rotationnumber of secondary dispersion in the step (B), it can be suppressedthat a dispersion medium is included by an aqueous monomer solutiondroplet, and accordingly the amount of the remaining dispersion mediumof water-absorbent resin can be more reduced.

The dispersion liquid obtained in the step (B) is subjected to a radicalpolymerization to obtain water-absorbent resin particles in a hydrousgel state, in which the water-absorbent resin is dispersed in thepetroleum hydrocarbon dispersion medium (step (C)).

Examples of the water-soluble radical polymerization initiator includepersulfates such as potassium persulfate, ammonium persulfate and sodiumpersulfate; peroxides such as hydrogen peroxide; and azo compounds suchas 2,2′-azobis(2-amidinopropane) dihydrochloride,2,2′-azobis[N-(2-carboxyethyl)-2-methylpropiondiamine]tetrahydrate,2,2′-azobis(1-imino-1-pyrrolidino-2-methylpropane) dihydrochloride and2,2′-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide].

Among them, potassium persulfate, ammonium persulfate, sodium persulfateand 2,2′-azobis(2-amidinopropane) dihydrochloride are preferred from aviewpoint of availability and easiness of handling.

The water-soluble radical polymerization initiator may be used incombination with reducing agents such as sulfite and ascorbic acid as aredox polymerization initiator.

The amount of the water-soluble radical polymerization initiator to beused is usually from 0.01 to 1 part by mass based on 100 parts by massof the water-soluble ethylenically unsaturated monomer. When the amountis less than 0.01 part by mass, a polymerization rate decreases, andwhen the amount is more than 1 part by mass, a rapid polymerizationreaction arises. Therefore, both cases are not preferred.

Timing of the addition of the water-soluble radical polymerizationinitiator is not limited in particular, but it is preferred topreviously add the water-soluble radical polymerization initiator to theaqueous solution of the water-soluble ethylenically unsaturated monomerfrom a viewpoint of homogeneity.

Moreover, upon polymerization, an internal-crosslinking agent may beadded to an aqueous solution of the monomer. Examples of theinternal-crosslinking agent include polyols such as (poly)ethyleneglycol [“(poly)” means a case where a prefix “poly” is attached or not,the same shall apply hereinafter], 1,4-butanediol, glycerol andtrimethylolpropane; polyunsaturated esters having two or more vinylgroups obtained by reacting polyols with an unsaturated acid such asacrylic acid or methacrylic acid; bisacrylamides such asN,N′-methylenebisacrylamide; and polyglycidyl compounds having two ormore glycidyl groups, such as (poly)ethylene glycol diglycidyl ether,(poly)ethylene glycol triglycidyl ether, (poly)glycerol diglycidylether, (poly)glycerol triglycidyl ether, (poly) propylene glycolpolyglycidyl ether and (poly)glycerol polyglycidyl ether. Theseinternal-crosslinking agents may be used alone, or two or more kinds ofthem may be used in combination.

The amount of the internal-crosslinking agent to be added is preferably3 parts by mass or less, more preferably 1 part by mass or less, andstill more preferably from 0.001 to 0.1 part by mass, based on 100 partsby mass of the water-soluble ethylenically unsaturated monomer. When theamount is more than 3 parts by mass, excess crosslinking arises andwater-absorption capability excessively deteriorates, and therefore itis not preferred.

It is preferred that the internal-crosslinking agent is previously addedto the aqueous solution of the water-soluble ethylenically unsaturatedmonomer.

The reaction temperature during reversed-phase suspension polymerizationin the present invention varies depending on the kind and amount of thepolymerization initiator to be used, and therefore cannot be determinedunconditionally. However, it is preferably from 20 to 100° C., and morepreferably from 40 to 90° C. When the reaction temperature is lower than20° C., the degree of polymelization may decrease, and when the reactiontemperature is higher than 100° C., a rapid polymerization reactionarises. Therefore, both cases are not preferred.

In the above-mentioned reversed-phase suspension polymerization,particles in a hydrous gel state which may become a water-absorbentresin with a small amount of remaining dispersion medium are obtained.Although these resultant particles in a hydrous gel state may bedehydrated, dried, and post-crosslinked to obtain water-absorbent resinparticles, it is advantageous that this polymerization is performed as afirst stage polymerization and second or subsequent stage ofpolymerization by further adding an aqueous monomer solution to a slurryof the dispersion medium containing a hydrous gel obtained by the firststage polymerization from viewpoints that a remaining dispersion mediareducing effect is enhanced, and that particle sizes of the resultantwater-absorbent resin is suitable for use in hygienic materials.

The size of particles after the first stage polymerization thus obtainedby polymerizing water-soluble ethylenically unsaturated monomers is amedian particle size preferably from 20 to 200 μm, more preferably from30 to 150 μm, and still more preferably from 40 to 100 μm, from aviewpoint of obtaining of a proper aggregated particle size inmulti-stage polymerization. Besides, the median particle size of polymerparticles after the first-stage polymerization is a value for particlesobtained by dehydration and drying after completion of the first stagepolymerization according to the measurement method described below.

The aqueous solution of the water-soluble ethylenically unsaturatedmonomer of the second stage polymerization is added to thepolymerization slurry obtained after completion of the step (C), andsubsequently, reversed-phase suspension polymerization at the secondstage is performed. Procedures are started from precipitations at leasta part of the above-mentioned surfactant after completion of the firststage polymerization.

Surfactants lose their essential capability to stabilize an aqueousphase droplet in an oil phase (or adversely, a capability to stabilizean oil phase droplet in an aqueous phase) when they precipitate.Examples of a precipitating method include, but not limited to, a methodof decreasing a temperature of slurry after polymerization by cooling.By precipitating at least a part of surfactants before adding theaqueous solution of a water-soluble ethylenically unsaturated monomer inthe second stage polymerization, droplets of the aqueous monomersolution added in the second stage polymerization is not stabilized in adispersion medium, and is absorbed in gel-like primary particles toenhance aggregation of the primary particles and thereby, a particlediameter suitable for use in hygienic materials is obtained. Inaddition, due to precipitation of the surfactants, generation of newO/W/O type droplets upon adding the aqueous monomer solution of thesecond stage polymerization is suppressed to prevent increase in anamount of the remaining dispersion medium. Therefore obtained waterabsorbent resin has lower amount of remaining dispersion medium thanthat involved in the first stage polymerization, because amount of waterabsorbent resin substantially increases through the second stagepolymerization which hardly increase in an amount of remainingdispersion medium. Besides, the hydrophobic polymeric dispersion agentdissolving together with the surfactants may precipitate in thedispersion medium upon adding the aqueous monomer solution in the secondstage because the dispersion agent becomes impossible to be dissolved ina dispersion medium by cooling.

After precipitating at least a part of the surfactant, the aqueoussolution of a water-soluble ethylenically unsaturated monomer in thesecond stage polymerization, which contains a water-soluble radicalpolymerization initiator, is stirred to mix to be absorbed andaggregated in the polymer gel in the first stage.

It is possible to use, as water-soluble ethylenically unsaturatedmonomers in the second-stage polymerization, the one similar to thoseexemplified as the water-soluble ethylenically unsaturated monomer inthe first stage polymerization. Kinds, neutralization degree andneutralized salt of the monomer, and the concentration of the aqueousmonomer solution may be the same as or different from those of thewater-soluble ethylenically unsaturated monomer in the first stagepolymerization.

The polymerization initiator to be added to an aqueous solution of awater-soluble ethylenically unsaturated monomer in the second stagepolymerization, any one may be selected from those exemplified as thepolymerization initiator used in the first stage polymerization to use.

If necessary, an internal-crosslinking agent and a chain transfer agentmay also be added to the aqueous solution of a water-solubleethylenically unsaturated monomer in the second stage polymerization,and any one may be selected from those exemplified for the first stagepolymerization to use.

The amount of the water-soluble ethylenically unsaturated monomer to beadded in the second stage polymerization is preferably from 1.0 to2.0-fold, and more preferably from 1.1 to 1.8-fold, based on the amountof the water-soluble ethylenically unsaturated monomer in the firststage polymerization from viewpoints of obtaining appropriate aggregatedparticles and reducing the amount of the remaining dispersion medium.When the amount of the water-soluble ethylenically unsaturated monomerto be added is less than 1.0-fold, the reduction effect by the amount ofthe remaining dispersion medium is low, being not preferable because theamount to be obtained decreases. When the amount of the water-solubleethylenically unsaturated monomer to be added is more than 2.0-fold,aggregated particles having a proper median particle size are notobtained, being not preferable because particles polymerized in thefirst stage polymerization cannot absorb fully the aqueous monomersolution in the second stage polymerization to cause fine powders.

It is sufficient that the entire components are mixed uniformly bystirring in the second stage of the reversed-phase suspensionpolymerization. The median particle size of aggregated particles may becontrolled depending on a precipitation state of the surfactants and aratio of the amount of the ethylenically unsaturated monomer in thesecond stage polymerization to the ethylenically unsaturated monomer inthe first stage polymerization.

Additionally, the median particle size of the aggregated particlessuitable for use in hygienic materials is preferably from 200 to 600 μm,more preferably from 250 to 500 μm, and still more preferably from 300to 450 μm.

After adding the aqueous monomer solution for a second stagepolymerization, polymerization is performed by means of a radicalpolymerization by warming

The reaction temperature in reversed-phase suspension polymerization inthe second stage polymerization cannot be determined unconditionallybecause it depends on the kind and amount of the polymerizationinitiator. However, it is preferably from 20 to 100° C., and morepreferably from 40 to 90° C.

At this time, when an amount of a petroleum hydrocarbon dispersionmedium used in the first stage polymerization is small or depending onthe kind of hydrophilic polymeric dispersion agent used, it may bepreferred that the dispersion medium is added to the polymerizationsystem after completion of the first stage polymerization. When theamount of a petroleum hydrocarbon dispersion medium used in thereversed-phase suspension polymerization is less than 50 parts by massbased on 100 parts by mass of the amount of the aqueous solution of awater-soluble ethylenically unsaturated monomer, it is not preferablebecause of difficulties in uniformly dispersion and temperature control.Therefore, if this balance breaks by adding an aqueous monomer solutionin the second stage polymerization, it is preferred to previously addthe dispersion medium after completion of the first stagepolymerization.

Moreover, in two-stage reversed-phase suspension polymerization usingpolyvinyl alcohol or polyglycerol as a hydrophilic polymeric dispersionagent shown here, it is preferred that a dispersion medium is added suchthat a W/O ratio calculated by dividing the total amount of the aqueousmonomer solution (the amount of the aqueous monomer solution in thefirst stage polymerization+the amount of the aqueous monomer solution inthe second stage polymerization) by the amount of a petroleumhydrocarbon dispersion medium, is less than 1.7, and more preferablyless than 1.4. When a W/O ratio is 1.7 or more, since an aggregation ofthe particles in the second stage polymerization tends to excessivelygenerate a water-absorbent resin having an optimal particle size may notbe obtained because their median particle size increases. Although themechanism is unclear, it is presumed that aggregation of particles ispromoted due to adherence of a hydrophilic polymeric dispersion agentsuch as polyvinyl alcohol and polyglycerol existing on the particlesurface, when the amount of the dispersion medium is insufficient.

The timing for adding the dispersion medium is not particularly limited,but it is preferred to add the dispersion medium at a low temperatureafter completion of the first stage polymerization as well asconsidering a cooling effect.

Furthermore, for the purpose of improving productivity, multi-stagereversed-phase suspension polymerization may be performed by performinga third or later stage polymerization reaction similar to the secondstage reversed-phase suspension polymerization.

After completion of these reversed-phase suspension polymerization, itis preferred to add a post-crosslinking agent containing two or morefunctional groups having reactivity with a functional group derived froma water-soluble ethylenically unsaturated monomer. The crosslinkingdensity of the surface layer of water-absorbent resin particles andvarious properties such as water-absorption capacity under load,water-absorption rate and gel strength can be enhanced by addingpost-crosslinking agent after the polymerization for reaction, and toimpart properties suitable for use in hygienic materials.

A post-crosslinking agent to be used in the post-crosslinking reactionis not particularly limited as long as it can react with a functionalgroup derived from the water-soluble ethylenically unsaturated monomerused in the polymerization.

Examples of the post-crosslinking agent to be used include polyols suchas ethylene glycol, propylene glycol, 1,4-butanediol,trimethylolpropane, glycerol, polyoxyethylene glycol, polyoxypropyleneglycol and polyglycerol; polyglycidyl compounds such as (poly)ethyleneglycol diglycidyl ether, (poly)ethylene glycol triglycidyl ether,(poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether,(poly) propylene glycol polyglycidyl ether and (poly)glycerolpolyglycidyl ether; haloepoxy compounds such as epichlorohydrin,epibromohydrin and α-methylepichlorohydrin; compound having two or morereactive functional groups, for example, isocyanate compounds such as2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetanecompounds such as 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol,3-ethyl-3-oxetane ethanol and 3-butyl-3-oxetane ethanol; oxazolinecompounds such as 1,2-ethylenebisoxazoline; and carbonate compounds suchas ethylene carbonate. These post-crosslinking agents may be used alone,or two or more kinds of them may be used in combination.

Among them, polyglycidyl compounds such as (poly)ethylene glycoldiglycidyl ether, (poly)ethylene glycol triglycidyl ether,(poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether,(poly)propylene glycol polyglycidyl ether and (poly)glycerolpolyglycidyl ether are preferred from a viewpoint of excellentreactivity.

The amount of the post-crosslinking agent to be added is preferably from0.01 to 5 parts by mass, and more preferably from 0.02 to 3 parts bymass, based on 100 parts by mass of the total amount of thewater-soluble ethylenically unsaturated monomer subjected to thepolymerization. When the amount of the post-crosslinking agent to beadded is less than 0.01 part by mass, it is impossible to enhancevarious properties such as water-absorption capacity under load,water-absorption rate and gel strength of the resultant water-absorbentresin, and when the amount to be added is more than 5 parts by mass,water-absorption capacity excessively deteriorates. Therefore both casesare not preferred.

The post-crosslinking agent may be added as it is, or added in a form ofan aqueous solution. If necessary, the post-crosslinking agent may beadded in a form of an aqueous solution containing a hydrophilic organicsolvent. Examples of the hydrophilic organic solvent include loweralcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol,isopropyl alcohol and propylene glycol; ketones such as acetone andmethyl ethyl ketone; ethers such as diethyl ether, dioxane andtetrahydrofuran; amides such as N,N-dimethylformamide; and sulfoxidessuch as dimethyl sulfoxide. These hydrophilic organic solvents may beused alone, or two or more kinds of them may be used in combination.Alternatively, these hydrophilic organic solvents may be used as a mixedsolvent with water.

The timing of the addition of the post-crosslinking agent may be aftercompletion of the polymerization and is not particularly limited. Thepost-crosslinking reaction is preferably performed in a dehydration ordrying step after the polymerization in the presence of water at anamount within a range from 1 to 200 parts by mass, more preferably from5 to 100 parts by mass, and still more preferably from 10 to 50 parts bymass, based on 100 parts by mass of the water-absorbent resin. Byadjusting the amount of water during the addition of thepost-crosslinking agent, post-crosslinking in the surface layer ofparticles of the water-absorbent resin can be more suitably performedand excellent water-absorption capability can be exhibited.

The temperature in the post-crosslinking reaction is preferably from 50to 250° C., more preferably from 60 to 180° C., still more preferablyfrom 60 to 140° C., and even more preferably from 70 to 120° C.

In the present invention, a drying step may be performed under a normalpressure or reduced pressure, or may be performed under a gas flow suchas nitrogen gas flow in order to enhance drying efficacy. When thedrying step is performed under a normal pressure, the drying temperatureis preferably from 70 to 250° C., more preferably from 80 to 180° C.,still more preferably from 80 to 140° C., and even more preferably from90 to 130° C. When the drying step is performed under reduced pressure,the drying temperature is preferably from 60 to 100° C., and morepreferably from 70 to 90° C.

The water content of the water-absorbent resin after drying is 20% orless, and preferably 10% or less, from a viewpoint of impartingfluidity. Inorganic lubricant agents such as an amorphous silica powderand the like may also be added to the water-absorbent resin so as toimprove fluidity.

EXAMPLES

The median particle size, the water content, and the amount of theremaining dispersion medium (amount of petroleum hydrocarbon dispersionmedium remaining in water-absorbent resin particles) of water-absorbentresins obtained in the respective Examples and Comparative Examples wereevaluated by the following methods.

Besides for the amount of the remaining dispersion medium described inPatent Document 4, since the absolute value of the amount of theremaining dispersion medium is decreased to 500 ppm or less, themeasuring method was improved in order to extract it more fully from thewater-absorbent resin.

(1) Median Particle Size

A water-absorbent resin(50 g) was passed through a JIS standard sievehaving a sieve opening size of 250 μm. The median particle size wasmeasured using a combination of sieves (α) when 50% by mass or more ofthe resin remaining on the sieve, while using a combination of sieves(β) when less than 50% by mass of the resin remaining on the sieve.

(α) JIS standard sieves were combined in a downward order of; a sievehaving a sieve opening size of 850 μm, a sieve having a sieve openingsize of 600 μm, a sieve having a sieve opening size of 500 μm, a sievehaving a sieve opening size of 425 μm, a sieve having a sieve openingsize of 300 μm, a sieve having a sieve opening size of 250 μm, a sievehaving a sieve opening size of 150 μm and a tray.

(β) JIS standard sieves were combined in a downward order of; a sievehaving a sieve opening size of 425 μm, a sieve having a sieve opening of250 μm, a sieve having a sieve opening size of 180 μm, a sieve having asieve opening size of 150 μm, a sieve with a sieve opening size of 106μm, a sieve with a sieve opening size of 75 μm, a sieve having a sieveopening size of 45 μm and a tray. About 50 g of the water-absorbentresin was placed on the uppermost sieve of the combination, andclassified for 20 minutes using a Rotap-type shaking machine.

After the sieve classification, the mass of the water-absorbent resinremaining on the respective sieves was calculated in terms of mass %based on the total mass of resin, the values were integrated in an orderfrom the resins with a larger particle size, and thereby the relationsbetween the sieve openings and integration values of the mass % of thewater-absorbent resin remaining on the sieve were plotted on alogarithmic-probability paper. The plots on the logarithmic-probabilitypaper were connected with a straight line, and the particle sizecorresponding to integrated mass % of 50% by mass was defined as themedian particle size.

(2) Water Content

About 2.5 g of the water-absorbent resin was accurately weighed (X g)into an aluminium cup, and after drying at 105° C. with a hot air dryerfor 2 hours, the mass of the dried water-absorbent resin was measured (Yg), and then the water content was calculated by the following equation.Besides, it is assumed that tare mass of the aluminium cup does notchange before and after drying.Water content(%)=(X−Y)/X×100(3) Amount of Remaining Dispersion Medium

In order to more fully extract the amount of the remaining dispersionmedium from the water-absorbent resin charged in the sample bottlerelative to the conventional measuring method of Patent Document 4 andthe like such that measurement of the amount of the remaining dispersionmedium at a lower level may be enabled, it is improved as follows suchthat swelling magnification is raised, and phosphoric acid is added toeasily dissolve a swelling gel.

(a) Formation of Calibration Curve

Approximate 10 g of the petroleum hydrocarbon dispersion medium(hereinafter referred to as a “dispersion medium”) used to polymerize asample for measuring a remaining dispersion medium, is placed into ascrew vial or the like to cool the vial with an ice-water bath.Similarly, 60 g of DMF (dimethylformamide) and 60 g of 25% by mass of anaqueous phosphoric acid solution is also cooled with an ice-water bath.(Charging is performed after sufficiently cooling because oftranspiration inhibition for the dispersion medium during charging.)

0.2 g of the above dispersion medium was accurately weighed into a 50 mlvolumetric screw vial and then the above cooled DMF was added thereto toaccurately make 20 g, followed by stirring with a magnetic stirrer barto obtain Standard sample solution 1. 0.2 g of the above Standard samplesolution 1 was then accurately weighed into a 50 ml volumetric screwvial and the above cooled DMF was added thereto to accurately make 20 g,followed by stirring with a magnetic stirrer bar to obtain Standardsample solution 2. This Standard sample solution 2 was also cooled withan ice-water bath.

In a 20 ml volumetric vial bottle (No. 5, manufactured by MaruemuCorporation), 0.02, 0.05, 0.1 or 0.5 g of the above Standard samplesolution 2 and 0.02 g of the above Standard sample solution 1 wereaccurately weighed and the above cooled DMF was added thereto to makethe amount of contents in each vial bottle to a total amount of 3.8 g (4ml). Furthermore, each vial bottle was charged with 5 ml of 25% by massof the aqueous phosphoric acid solution, sealed and tightened with aseptum rubber and an aluminium cap, and then stirring was performed byshaking each the bottle.

In addition, attention has been paid to perform quickly procedures fromcharging of the sample into the 20-ml volumetric vial to the sealing, toprevent a dispersion medium from transpiring from the vial as possible.Moreover, attention has been paid also to fully cool DMF and 25% by massof an aqueous phosphoric acid solution such that the dispersion mediumdid not transpire due to generation of heat at the time of mixing theboth reagents, and to fully mix them after sealing with an aluminium capor the like.

This vial bottle was warmed at 110° C. for 2 hours, and 1 ml of a vaporphase portion was collected such that the vapor phase portion was notcooled, and then it was injected into a gas chromatograph to obtain achromatogram.

(Use of Head Space Autosampler)

The concentrations of the above Standard sample solutions werecalculated based on amount to be charged, and then the amount to becharged of the dispersion medium in each vial bottle was calculated toprepare a calibration curve based on the amount to be charged and a peakarea of the chromatogram. (When a mixture of petroleum hydrocarbons wasused as the dispersion medium, plural peaks appeared and therefore acalibration curve was prepared based on a total value of the areas andthe amount to be charged).

(b) Measurement of Amount of Dispersion Medium Remaining in Sample

About 2 g of a sample of the water-absorbent resin to be measured wascharged into an aluminum cup and then dried with a hot air dryer at 105°C. for 2 hours to adjust the water content.

Required amounts of DMF and 25% by mass of an aqueous phosphoric acidsolution used for the measurement, were also charged into a screwbottle, and cooled with an ice-water bath.

In a 20 ml volumetric vial bottle (No. 5, manufactured by MaruemuCorporation), each of 0.01 g of the above sample was accurately weighed,and after attaching a tentative rubber cap the bottom of the vial bottlewas dipped in an ice bath to cool the vial bottle and thewater-absorbent resins. To this vial bottle were added 4 ml of the abovecooled DMF and, further 5 ml of 25% by mass of the above cooled aqueousphosphoric acid solution. The vial bottle was quickly tightened bysealing with a septum rubber and an aluminium cap and, then gentlyshaken to mix. After allowing to stand for 10 min, it was confirmed thatthe water-absorbent resin in the vial bottle was swelled, the vialbottle was vigorously shaken to agitate the inside strongly. This vialbottle was pre-heated at 110° C. for 2 hours to strongly agitate theinside again after heating.

In addition, attention has been paid to perform quickly procedures fromcharging of the sample into the 20-ml volumetric vial to the sealing, toprevent a dispersion medium from transpiring from the vial as possibleas much as possible.

This vial bottle was warmed at 110° C. for 2 hours, and 1 ml of a vaporphase portion was collected such that the vapor phase portion was notcooled, and then it was injected into a gas chromatograph to obtain achromatogram.

(Use of Head Space Autosampler)

The amount of the dispersion medium contained in the amount (0.10 g ofobserved values) of the charged sample was calculated from thecalibration curve made based on the peak area of the resultantchromatogram, and then converted into the amount [ppm] of the dispersionmedium contained per 1 g of the sample.

The conditions of a gas chromatograph used in the measurement of theamount of the remaining dispersion medium in the present invention areas follows.

Model: GC-14A+HSS2B (HEADSPACE Autosampler) manufactured by ShimadzuCorporation

Filler: Squalane 25% Shimalite (NAW) (101)

-   -   80-100 mesh

Column: 3.2 mm in diameter×2 m

Column temperature: 80° C.

Injection port temperature: 180° C.

Detector temperature: 180° C.

Detector: FID

Gas carrier: Nitrogen gas

Vial bottle heating temperature: 110° C.

Syringe setting temperature: 110° C.

Moreover, each of Examples and Comparative Examples was performed 3times, and the amount of the remaining dispersion medium of each groupwas shown as Mean±Standard deviation. Statistical evaluation ofdifferences between each groups was performed using Student's t-test (*shows p<0.05).

(c) Comparison with Conventional Measuring Method

As a result of measuring the amount of the remaining dispersion mediumfor the same sample, the above-mentioned measuring method of the presentapplication could have measured the amount of the remaining dispersionmedium by high sensitivity more as compared with the conventional methodof Patent Document 4. For example, for Comparative Example 1 of thepresent application, while the measuring method of the presentapplication exhibits 100 ppm of the amount of the remaining dispersionmedium, the conventional method inhibits only 80 ppm of the amount ofthe remaining dispersion medium.

(d) Measurement of Amount of Dispersion Medium Dissolved in AqueousMonomer Solution in the Absence of Surfactants and the Like

The following experiment was conducted for the purpose of investigatingthe amount of dispersion medium dissolved in an aqueous monomer solutionin the absence of surfactants and the like.

Reference Experimental Example

A measurement of a dissolved amount of a dispersion medium in an aqueousmonomer solution was performed by the following procedures:

1) Into a 500 mL Erlenmeyer flask, 46.0 g of 80% by mass of acrylic acidwas charged and neutralized by adding dropwise 51.1 g of 30% by masssodium hydroxide under stirring while cooling the flask from theoutside. To this were added 21.9 g of ion exchange water to prepare anaqueous solution of a water-soluble ethylenically unsaturated monomer.(the aqueous monomer solution having a neutralization degree of 75 mol %and a concentration of 38% by mass)

2) In a 2 L volumetric five-necked cylindrical separable round-bottomflask (hereinafter referred to as a “round-bottom flask”) equipped witha stirrer with two steps of 50 mm in diameter pitched blade paddleimpellers, a thermometer and a condenser, 171 g of n-heptane wasweighted.

3) The above-mentioned round bottom flask was dipped in a water bath,and n-heptane was agitated at 500 rpm, and maintained to an insidetemperature of 40±1° C.

4) The acrylic acid neutralization aqueous solution prepared inProcedure 1) was supplied, and the temperature of the water bath wasadjusted to maintain the temperature such that it becomes an insidetemperature of 40±1° C. while stirring at 500 rpm for 30 minutes.

5) After stirring for 30 minutes, the stirrer is stopped, and the roundbottom flask is allowed to stand for 30 minutes while maintaining thesame water bath temperature.

6) Only lower layer neutralization liquid layer was gently withdrawnsuch that two-layer separation is not mixed.

7) According to the measuring method of the amount of remainingdispersion medium, about 0.26 g (corresponding to about 0.1 g in theamount of the monomer) of the neutralized liquid withdrawn in 20 mLvolumetric vial bottle was accurately weighted to add cooled DMF andphosphoric acid solution.

8) After sealing with a vial cap and stirring, preheating at 110° C. for2 hours is performed, and according to the measuring method of theamount of remaining dispersion medium, the amount of n-heptane in theneutralized liquid was measured.

As experimental results, the same conditions as in the first stagepolymerization described in the present application, namely, the amountof n-heptane (dispersion medium) dissolved in the acrylic acidneutralization aqueous solution (aqueous monomer solution) at 40° C.,was 80 ppm on the monomer mass basis. Therefore, this amount (80 ppm) ofthe dispersion medium was considered to be the minimum amount of theremaining dispersion medium which can be reduced by the conventionalmethod (in Patent Document 4 and the like).

The present invention will be described in detail by way of Examples,but the present invention is not limited only to these Examples.

Comparative Example 1

The example 8 of WO2009/025235 (Patent document 4) was performed asComparative Example 1. In addition, the example is an example with thelowest amount of the remaining dispersion medium in Patent Document 4.

Into a 500 mL Erlenmeyer flask, 92.0 g of 80% by mass of acrylic acidwas charged and neutralized by adding dropwise 102.2 g of 30% by masssodium hydroxide under stirring while cooling the flask from theoutside. To this were added 0.11 g of potassium persulfate, 8.3 mg ofethylene glycol diglycidyl ether and 43.6 g of ion-exchange water toprepare an aqueous solution of a water-soluble ethylenically unsaturatedmonomer.

In a 2 L volumetric five-necked cylindrical round-bottom separable flask(hereinafter referred to as a “round-bottom flask”) equipped with astirrer having a two-stage pitched blade paddle impellers of 50 mm indiameter, a thermometer, a reflux condenser and a nitrogen gasintroducing tube, 334 g of n-heptane was weighted as a petroleumhydrocarbon dispersion medium. To the round-bottom flask were added 0.46g of an oxidized ethylene-propylene copolymer (manufactured by MitsuiChemicals, Inc., trade name: HIWAX 4052E) and 0.46 g of a maleicanhydride-modified ethylene-propylene copolymer (manufactured by MitsuiChemicals, Inc., trade name: HIWAX 1105A) as a hydrophobic polymericdispersion agent, and then this was warmed up to 83 to 86° C. with awater bath at 90° C. at a stirring rate of 300 rpm to dissolve anddisperse it before air cooling to an inner temperature of 61° C. Theabove-mentioned aqueous monomer solution was charged once into the aboveheptane while stirring them at 300 rpm by using a funnel made of SUS,having an inside diameter of 8 mm at opening at the tip. After addingthe aqueous monomer solution, it was agitated at an inner temperature of40° C. for 10 minutes and primarily dispersed.

Next, a solution separately prepared by warming 0.92 g of a sucrosefatty acid ester (manufactured by Mitsubishi-Kagaku Foods Corporation,trade name: S-370) as a surfactant to dissolve it in 8.28 g of n-heptaneby warming at 60° C. or higher was added to the round-bottom flaskthrough a funnel and then a stirring rate was increased to 500 rpm tosecondly disperse the aqueous monomer solution.

The atmosphere in the system was well substituted with nitrogen whilemaintaining the inner temperature of the round-bottom flask containingthe dispersion at 40° C., and a radical polymerization reaction wasperformed by warming for 1 hour with a hot water bath at 70° C.

After completion of the first stage polymerization, the stirring ratewas increased to 1,000 rpm and the inner temperature was lowered to nearroom temperature to precipitate at least a part of the surfactant.

Separately, to a 500 mL Erlenmeyer flask, 128.8 g of 80 mass % acrylicacid was added and neutralized by adding dropwise 142.9 g of 30 mass %sodium hydroxide under stirring while cooling the flask from theoutside. To this were added 0.15 g of potassium persulfate, 11.6 mg ofethylene glycol diglycidyl ether and 16.7 g of distilled water toprepare an aqueous monomer solution for the second stage polymerization.Next, the aqueous monomer solution for the above-mentioned second stagewas added to the above-mentioned cooled polymerization slurry throughthe dropping funnel, and stirred to mix for some time, and absorbed intoa polymerization gel in the first stage to aggregate hydrous gelparticles. Subsequently, the atmosphere in the system was wellsubstituted with nitrogen while maintaining the inner temperature of theround-bottom flask near room temperature, and a radical polymerizationreaction was performed by warming for 1 hour with a hot water bath at70° C.

After the polymerization reaction in the second stage, the reactionsuspension was heated using an oil bath at 120° C. and about 260 g ofwater was removed off from the system by azeotropic distillation whilerefluxing heptane in the flask to obtain a dehydrated polymer dispersedin heptane. To the resultant heptane dispersed dehydrated polymer, 8.2 gof a 2% aqueous solution of ethylene glycol diglycidyl ether as apost-crosslinking agent was added and the post-crosslinking reaction wasperformed at 83° C. for 2 hours.

Then, heating is performed using an oil bath at 120° C., heptane andwater were removed off from the system by distillation, followed bydrying under a nitrogen gas flow to obtain 235 g of a water-absorbentresin having in a form of aggregated spherical particles by passingthrough a sieve of 850 μm. The median particle size of thiswater-absorbent resin was 356 μm, and water content was 4.5%.

Comparative Example 2

The example 7 of WO2009/025235 (Patent document 4) was performed asComparative Example 2.

According to the same manner as that of Comparative Example 1, exceptthat in Comparative Example 1, 0.92 g of a maleic anhydride-modifiedethylene-propylene copolymer (manufactured by Mitsui Chemicals, Inc.,trade name: HIWAX 1105A) were added substituted for 0.46 g of anoxidized ethylene-propylene copolymer (manufactured by Mitsui Chemicals,Inc., trade name: HIWAX 4052E) and 0.46 g of a maleic anhydride-modifiedethylene-propylene copolymer (manufactured by Mitsui Chemicals, Inc.,trade name: HIWAX 1105A) as a hydrophobic polymeric dispersion agent,and this was warmed up to 75 to 80° C. with a water bath at 85° C. todissolve and disperse it in the dispersion medium, 237 g of awater-absorbent resin having in a form of aggregated spherical particleswas obtained. This water-absorbent resin had a median particle size of372 μm, and a water content of 4.8%.

Example 1

In Comparative Example 1 (Patent Document 4, Example 8), polyvinylpyrrolidone (hereinafter referred to as “PVP”) as a hydrophilicpolymeric dispersion agent was added to an aqueous monomer solution forthe first stage polymerization at 1.0% by mass based on a mass of theaqueous monomer solution.

50 g of 5% by mass aqueous solution of PVP (manufactured by ISP JapanCo. Ltd.: K-90) was prepared prior to the preparation of an aqueousmonomer solution. Subsequently, into a 500 mL Erlenmeyer flask, 92.0 gof 80% by mass of acrylic acid was charged and neutralized by addingdropwise 102.2 g of 30% by mass sodium hydroxide under stirring whilecooling the flask from the outside. To this were added 0.11 g ofpotassium persulfate, 8.3 mg of ethylene glycol diglycidyl ether, 18.1 gof the above-mentioned 5 mass % PVP aqueous solution and 25.5 g of ionexchange water to prepare, and stir to mix an aqueous monomer solution.

In a 2 L volumetric five-necked round-bottom flask equipped with astirrer having a two-stage pitched blade paddle impellers of 50 mm indiameter, a thermometer, a reflux condenser and a nitrogen gasintroducing tube, 334 g of heptane was weighted as a petroleumhydrocarbon dispersion medium. To the round-bottom flask were added 0.46g of an oxidized ethylene-propylene copolymer (manufactured by MitsuiChemicals, Inc., trade name: HIWAX 4052E) and 0.46 g of a maleicanhydride-modified ethylene-propylene copolymer (manufactured by MitsuiChemicals, Inc., trade name: HIWAX 1105A) as a hydrophobic polymericdispersion agent, and then this was warmed up to 83 to 86° C. with awater bath at 90° C. at a stirring rate of 300 rpm to dissolve anddisperse it before air cooling to an inner temperature of 61° C. Theabove-mentioned aqueous monomer solution was charged once into the aboveheptane while stirring them at 300 rpm by using a funnel made of SUS,having an inside diameter of 8 mm at opening at the tip. After addingthe aqueous monomer solution, it was agitated at an inner temperature of40° C. for 10 minutes and primarily dispersed.

Next, a solution separately prepared by warming 0.92 g of a sucrosefatty acid ester (manufactured by Mitsubishi-Kagaku Foods Corporation,trade name: S-370) as a surfactant to dissolve it in 8.28 g of n-heptaneby warming at 60° C. or higher was added to the round-bottom flaskthrough a funnel and then a stirring rate was increased to 500 rpm tosecondly disperse the aqueous monomer solution. The atmosphere in thesystem was well substituted with nitrogen while maintaining the innertemperature of the round-bottom flask containing the dispersion at 40°C., and a radical polymerization reaction was performed by warming for 1hour with a hot water bath at 70° C.

After completion of the first stage polymerization, the stirring ratewas increased to 1,000 rpm and the inner temperature was cooled to 30°C. or less to precipitate at least a part of the surfactant.

Separately, to a 500 mL Erlenmeyer flask, 128.8 g of 80 mass % acrylicacid was added and neutralized by adding dropwise 142.9 g of 30 mass %sodium hydroxide under stirring while cooling the flask from theoutside. To this were added 0.15 g of potassium persulfate, 11.6 mg ofethylene glycol diglycidyl ether and 16.7 g of distilled water toprepare an aqueous monomer solution in the second stage polymerization.Next, the aqueous monomer solution for the above-mentioned second stagepolymerization was added to the above-mentioned cooled polymerizationslurry through a dropping funnel, stirred to mix for some time, andabsorbed into a polymerization gel in the first stage to aggregatehydrous gel particles. Then, the atmosphere in the system was wellsubstituted with nitrogen while maintaining the inner temperature of theround-bottom flask, and a radical polymerization reaction was performedby warming for 1 hour with a hot water bath at 70° C.

After the polymerization reaction in the second stage, the reactionsuspension was heated using an oil bath at 120° C. and 260 g of waterwas removed off from the system by azeotropic distillation whilerefluxing heptane in the flask to obtain a dehydrated polymer dispersedin heptane. To the resultant heptane dispersed dehydrated polymer, 8.2 gof a 2% aqueous solution of ethylene glycol diglycidyl ether as apost-crosslinking agent was added and the post-crosslinking reaction wasperformed at 83° C. for 2 hours.

Then, heating is performed using an oil bath at 120° C., heptane andwater were removed off from the system by distillation, followed bydrying under a nitrogen gas flow to obtain 228 g of a water-absorbentresin having in a form of aggregated spherical particles by passingthrough a sieve of 850 μm. This water-absorbent resin had a medianparticle size of 387 μm, and a water content of 5.1%.

Example 2

In Example 1, PVP as a hydrophilic polymeric dispersion agent was addedto an aqueous monomer solution for the first stage polymerization at2.0% by mass based on a mass of the aqueous monomer solution.

50 g of 5% by mass aqueous solution of PVP (manufactured by ISP JapanCo. Ltd.: K-90) was prepared prior to the preparation of an aqueousmonomer solution. Subsequently, into a 500 mL Erlenmeyer flask, 92.0 gof 80% by mass of acrylic acid was charged and neutralized by addingdropwise 102.2 g of 30% by mass sodium hydroxide under stirring whilecooling the flask from the outside. To this were added 0.11 g ofpotassium persulfate, 8.3 mg of ethylene glycol diglycidyl ether, 36.2 gof the above-mentioned 5 mass % PVP aqueous solution and 7.4 g of ionexchange water to prepare, and stir to mix an aqueous monomer solution.

In a 2 L volumetric five-necked round-bottom flask equipped with astirrer having a two-stage pitched blade paddle impellers of 50 mm indiameter, a thermometer, a reflux condenser and a nitrogen gasintroducing tube, 334 g of heptane was weighted as a petroleumhydrocarbon dispersion medium. To the round-bottom flask were added 0.46g of an oxidized ethylene-propylene copolymer (manufactured by MitsuiChemicals, Inc., trade name: HIWAX 4052E) and 0.46 g of a maleicanhydride-modified ethylene-propylene copolymer (manufactured by MitsuiChemicals, Inc., trade name: HIWAX 1105A) as a hydrophobic polymericdispersion agent, and then this was warmed up to 75 to 80° C. with awater bath at 85° C. at a stirring rate of 300 rpm to dissolve anddisperse it before air cooling to an inner temperature of 61° C. Theabove-mentioned aqueous monomer solution was charged once into the aboveheptane while stirring them at 300 rpm by using a funnel made of SUS,having an inside diameter of 8 mm at opening at the tip. After addingthe aqueous monomer solution, it was agitated at an inner temperature of40° C. for 10 minutes and primarily dispersed.

Next, a solution separately prepared by warming 0.92 g of a sucrosefatty acid ester (manufactured by Mitsubishi-Kagaku Foods Corporation,trade name: S-370) as a surfactant to dissolve it in 8.28 g of n-heptaneby warming at 60° C. or higher was added to the round-bottom flaskthrough a funnel and then a stirring rate was increased to 700 rpm tosecondly disperse the aqueous monomer solution.

Subsequently, according to the same manner as that of Example 1, 225 gof a water-absorbent resin having in a form of aggregated sphericalparticles was obtained. This water-absorbent resin had a median particlesize of 420 μm, and a water content of 4.5%.

Example 3

To the aqueous monomer solution for the first stage polymerization, 0.7%by mass of polyvinyl alcohol (hereinafter abbreviated to “PVA”) as ahydrophilic polymeric dispersion agent was used based on an amount ofthe monomer.

20 g of PVA (manufactured by Nippon Synthetic Chemical Industry Co.,Ltd.: Gosenol GH-20) was heated, stirred, dissolved in 380 g ofdistilled water, and allowed to stand to prepare 400 g of 5% by massaqueous solution of PVA. Subsequently, into a 500 mL Erlenmeyer flask,92.0 g of 80% by mass of acrylic acid was charged and neutralized byadding dropwise 102.2 g of 30% by mass sodium hydroxide under stirringwhile cooling the flask from the outside. To this were added 0.07 g ofpotassium persulfate, 10.1 mg of ethylene glycol diglycidyl ether, 12.7g of the above-mentioned 5 mass % PVA aqueous solution as a hydrophilicpolymeric dispersion agent and 30.9 g of ion exchange water to preparean aqueous monomer solution.

In a 2 L volumetric five-necked cylindrical round-bottom separable flaskequipped with a stirrer having a two-stage pitched blade paddleimpellers of 50 mm in diameter, a thermometer, a reflux condenser and anitrogen gas introducing tube, 275 g of heptane was weighted as apetroleum hydrocarbon dispersion medium. To the round-bottom flask wasadded 0.74 g of a maleic anhydride-modified ethylene-propylene copolymer(manufactured by Mitsui Chemicals, Inc., trade name: HIWAX 1105A) as ahydrophobic polymeric dispersion agent, and then this was warmed up to75 to 80° C. with a water bath at 85° C. at a stirring rate of 300 rpmto dissolve and disperse it before air cooling to an inner temperatureof 65° C. The above-mentioned aqueous monomer solution was charged onceinto the above heptane while stirring them at 300 rpm by using a funnelmade of SUS, having an inside diameter of 8 mm at opening at the tip.After adding the aqueous monomer solution, it was agitated at an innertemperature of 40° C. for 10 minutes and primarily dispersed.

Next, a solution separately prepared by warming 0.74 g of a sucrosefatty acid ester (manufactured by Mitsubishi-Kagaku Foods Corporation,trade name: S-370) as a surfactant to dissolve it in 6.6 g of n-heptaneby warming at 60° C. or higher was added to the round-bottom flaskthrough a funnel and then a stirring rate was increased to 500 rpm tosecondly disperse the aqueous monomer solution.

The atmosphere in the system was well substituted with nitrogen whilemaintaining the inner temperature of the round-bottom flask containingthe dispersion at 40° C., and a radical polymerization reaction wasperformed by warming for 1 hour with a hot water bath at 70° C.

After completion of the first stage polymerization, the stirring ratewas increased to 1,000 rpm, and the inner temperature was cooled to near30° C. to precipitate at least a part of the surfactant.

Separately, to a 500 mL Erlenmeyer flask, 101.2 g of 80 mass % acrylicacid was added and neutralized by adding dropwise 112.4 g of 30 mass %sodium hydroxide under stirring while cooling the flask from theoutside. To this were added 0.08 g of potassium persulfate, 9.1 mg ofethylene glycol diglycidyl ether and 12.6 g of distilled water toprepare an aqueous monomer solution in the second stage polymerization.Next, the aqueous monomer solution for the above-mentioned second stagepolymerization was added to the above-mentioned cooled polymerizationslurry through a dropping funnel, stirred to mix for some time, andabsorbed into a polymerization gel in the first stage to aggregatehydrous gel particles. Subsequently, the atmosphere in the system waswell substituted with nitrogen while maintaining the inner temperatureof the round-bottom flask near room temperature, and a radicalpolymerization reaction was performed by warming for 1 hour with a hotwater bath at 70° C.

After the second stage of polymerization reaction, 100 g of heptane wasadded, followed by heating it with a 120° C. oil bath, and thedehydration polymer dispersed by heptane was obtained by removing 220 gof water out of a system by azeotropic distillation, refluxing heptanein a flask. To the resultant heptane dispersed dehydrated polymer, 3.9 gof a 2% aqueous solution of ethylene glycol diglycidyl ether as apost-crosslinking agent was added and the post-crosslinking reaction wasperformed at 83° C. for 2 hours.

Then, heating is performed using an oil bath at 120° C., heptane andwater were removed off from the system by distillation, followed bydrying under a nitrogen gas flow to obtain 185 g of a water-absorbentresin having in a form of aggregated spherical particles by passingthrough a sieve of 850 μm. This water-absorbent resin had a medianparticle size of 355 μm, and a water content of 6.2%.

Example 4

To the aqueous monomer solution for the first stage polymerization, 0.8%by mass of polyvinyl alcohol (hereinafter abbreviated to “PVA”) based onan amount of the aqueous monomer solution was used as a hydrophilicpolymeric dispersion agent.

Into a 500 mL Erlenmeyer flask used, 92.0 g of 80% by mass of acrylicacid was charged and neutralized by adding dropwise 102.2 g of 30% bymass sodium hydroxide under stirring while cooling the flask from theoutside. To this were added 0.07 g of potassium persulfate, 10.1 mg ofethylene glycol diglycidyl ether, 14.7 g of the above-mentioned 5 mass %PVA aqueous solution identical to in Example 3 and 28.9 g of ionexchange water to prepare an aqueous solution of the water-solubleethylenically unsaturated monomer to prepare, and stir to mix an aqueousmonomer solution.

In a 2 L volumetric five-necked cylindrical round-bottom separable flaskequipped with a stirrer having a two-stage pitched blade paddleimpellers of 50 mm in diameter, a thermometer, a reflux condenser and anitrogen gas introducing tube, 275 g of heptane was weighted as apetroleum hydrocarbon dispersion medium. To the round-bottom flask wasadded 0.74 g of a maleic anhydride-modified ethylene-propylene copolymer(manufactured by Mitsui Chemicals, Inc., trade name: HIWAX 1105A) as ahydrophobic polymeric dispersion agent, and then this was warmed up to75 to 80° C. with a water bath at 85° C. at a stirring rate of 300 rpmto dissolve and disperse it before air cooling to an inner temperatureof 65° C. The above-mentioned aqueous monomer solution was charged onceinto the above heptane while stirring them at 300 rpm by using a funnelmade of SUS, having an inside diameter of 8 mm at opening at the tip.After adding the aqueous monomer solution, it was agitated at an innertemperature of 40° C. for 10 minutes and primarily dispersed.

Next, a solution separately prepared by warming 0.74 g of a sucrosefatty acid ester (manufactured by Mitsubishi-Kagaku Foods Corporation,trade name: S-370) as a surfactant to dissolve it in 6.6 g of n-heptaneby warming at 60° C. or higher was added to the round-bottom flaskthrough a funnel and then a stirring rate was increased to 700 rpm tosecondly disperse the aqueous monomer solution.

The atmosphere in the system was well substituted with nitrogen whilemaintaining the inner temperature of the round-bottom flask containingthe dispersion at 40° C., and a radical polymerization reaction wasperformed by warming for 1 hour with a hot water bath at 70° C.

After completion of the first stage polymerization, 141 g of heptane wasadded, the stirring rate was increased to 1,000 rpm, and then the innertemperature was decreased to near 30° C. to precipitate at least a partof the surfactant.

Separately, to a 500 mL Erlenmeyer flask, 128.8 g of 80 mass % acrylicacid was added and neutralized by adding dropwise 142.9 g of 30 mass %sodium hydroxide under stirring while cooling the flask from theoutside. To this were added 0.10 g of potassium persulfate, 11.6 mg ofethylene glycol diglycidyl ether and 16.7 g of distilled water toprepare an aqueous monomer solution for the second stage polymerization.Next, to the above-mentioned second stage polymerization was added theaqueous monomer solution of the above-mentioned second stagepolymerization through a dropping funnel, are stirred to mix for sometime, dispersed, and absorbed into a polymerization gel in the firststage to aggregate hydrous gel particles. Subsequently, the atmospherein the system was well substituted with nitrogen while maintaining theinner temperature of the round-bottom flask near room temperature, and aradical polymerization reaction was performed by warming for 1 hour witha hot water bath at 70° C.

After the polymerization reaction in the second stage, the reactionsuspension was heated using an oil bath at 120° C. and about 250 g ofwater was removed off from the system by azeotropic distillation whilerefluxing heptane in the flask to obtain a dehydrated polymer dispersedin heptane. To the resultant heptane dispersed dehydrated polymer, 4.4 gof a 2% aqueous solution of ethylene glycol diglycidyl ether as apost-crosslinking agent was added and the post-crosslinking reaction wasperformed at 83° C. for 2 hours.

Then, heating is performed using an oil bath at 120° C., heptane andwater were removed off from the system by distillation, followed bydrying under a nitrogen gas flow to obtain 223 g of a water-absorbentresin having in a form of aggregated spherical particles by passingthrough a sieve of 850 μm. This water-absorbent resin had a medianparticle size of 378 μm, and a water content of 5.9%.

Comparative Example 3

In Example 1, two-step dispersion (primary and secondary dispersion) ofthe aqueous monomer solution for the first stage polymerization, whichis the feature of the present invention, was not performed asComparative Example 3.

Specifically, after preparing an aqueous monomer solution for the firststage polymerization according to the same manner as that of Example 1,342 g of n-heptane as a petroleum hydrocarbon dispersion medium wasweighted in a 2 L volumetric six-necked round-bottom flask equipped witha stirrer having a two-stage pitched blade paddle impellers of 50 mm indiameter, a thermometer, a reflux condenser and a nitrogen gasintroducing tube. To the round-bottom flask were added 0.92 g of asucrose fatty acid ester (manufactured by Mitsubishi-Kagaku FoodsCorporation, trade name: S-370) as a surfactant, and 0.46 g of anoxidized ethylene-propylene copolymer (manufactured by Mitsui Chemicals,Inc., trade name: HIWAX 4052E) and 0.46 g of a maleic anhydride-modifiedethylene-propylene copolymer (manufactured by Mitsui Chemicals, Inc.,trade name: HIWAX 1105A) as a polymeric dispersion agent, and then thiswas warmed up to 83 to 86° C. with a water bath at 90° C. at a stirringrate of 300 rpm to dissolve and disperse it before the stirring rate wasincreased up to 300 rpm to air cool it to an inner temperature of 61° C.The above-mentioned aqueous monomer solution was added by using a funnelmade of SUS, having an inside diameter of 8 mm at opening at the tip,and dispersed while stirring them at 300 rpm. According to the samemanner as that of Example 1, except that a surfactant is not added, itwas stirred at 300 rpm for 10 minutes, and then the stirring rate wasincreased to 500 rpm. The atmosphere in the system was well substitutedwith nitrogen while maintaining the inner temperature of theround-bottom flask at 40° C., and a radical polymerization reaction wasperformed by warming for 1 hour with a hot water bath at 70° C.

Subsequently, according to the same manner as that of Example 1, 225 gof a water-absorbent resin having in a form of aggregated sphericalparticles was obtained. This water-absorbent resin had a median particlesize of 389 μm, and a water content of 4.8%.

Comparative Example 4

To the aqueous monomer solution for the first stage polymerization inComparative Example 2 was added 0.3% by mass of hydroxyethyl cellulose(hereinafter abbreviated to “HEC”) based on an amount of the aqueousmonomer solution, as a thickener.

Into a 500 mL Erlenmeyer flask, 92.0 g of 80% by mass of acrylic acidwas charged and neutralized by adding dropwise 102.2 g of 30% by masssodium hydroxide under stirring while cooling the flask from theoutside. To this neutralization liquid, 0.27 g of HEC (manufactured bySumitomo Seika Chemicals Co., Ltd.: AW-15F) was added to fully agitateand dissolve it. To this were added 0.11 g of potassium persulfate, 8.3mg of ethylene glycol diglycidyl ether and 43.3 g of ion exchange waterto prepare an aqueous monomer solution.

Subsequently, an experiment was carried out according to the same manneras that of Example 1, and 226 g of a water-absorbent resin having in aform of aggregated spherical particles was obtained. Thiswater-absorbent resin had a median particle size of 378 μm, and a watercontent of 5.2%.

Comparative Example 5

According to the same manner as that of Comparative Example 4, exceptthat the amount of HEC to be added was changed to 0.91 g whichcorresponds to 1.0% by mass and the amount of ion exchange water waschanged to 42.7 g, an experiment was carried out. About 70 percent inthe whole of the resultant particles is a particle having a particlesize of 850 μm or more, and there is 86 g of particles having a particlesize of 850 μm or, and their water content was 6.2%. An amount of aremaining dispersion medium was measured by this particle of 850 μm orless.

Each of Examples 1-4, and Comparative Examples 1-5 was carried out threetimes. For each of the resultant water-absorbent resins, an amount ofthe remaining dispersion medium was measured by using a measuring methodof measuring an amount of a remaining dispersion medium, Mean±Standarddeviation for them are shown in Table 1.

TABLE 1 Amount of Hydrophilic Hydrophobic Stirring rate remainingpolymeric polymeric [rpm] dispersion dispersion dispersion PrimarySecondary medium Examples agent agent dispersion dispersion [ppm]Example 1 PVP 1.0% Oxidized PE- 300 500  61 ± 3.6* PP + maleic modifiedPE- PP Example 2 PVP 2.0% Oxidized PE- 300 700  71 ± 4.2* PP + maleicmodified PE- PP Example 3 PVA 0.7% maleic 300 500  54 ± 3.1* modifiedPE- PP Example 4 PVA 0.8% maleic 300 500  83 ± 4.7* modified PE- PPComparative Example 1 — Oxidized PE- 300 500 104 ± 6.0 PP + maleicmodified PE- PP Comparative Example 2 — maleic 300 500 353 ± 9.8modified PE- PP Comparative Example 3 PVP 1% Oxidized PE- 300/500  317 ±13.6 PP + maleic (Without two steps modified PE- dispersion) PPComparative Example 4 HEC 0.3% maleic 300 500  372 ± 13.5 anhydride-modified PE- PP Comparative Example 5 HEC 1.0% maleic 300 500 1257 ±80.7 modified PE- PP Amount of the remaining dispersion medium is shownby Mean ± Standard deviation. *P < 0.05 (vs. Comparative Examples 1-5)

INDUSTRIAL APPLICABILITY

The present invention provides a method for producing a water-absorbentresin, wherein an amount of a remaining petroleum hydrocarbon dispersionmedium used in a reversed phase suspension polymerization, which iscontained in the water-absorbent resin and an odor originated form thepetroleum hydrocarbon dispersion medium is further reduced, and awater-absorbent resin obtained by the method.

The invention claimed is:
 1. A method for producing a water-absorbentresin comprising at least two stages of reversed-phase suspensionpolymerization, wherein the first stage of the at least two stages ofreversed-phase suspension polymerization comprises the following stepsof: (A) performing a primary dispersion by stirring to mix in theabsence of surfactants an aqueous solution of a water-solubleethylenically unsaturated monomer containing a hydrophilic polymericdispersion agent with a petroleum hydrocarbon dispersion medium in whicha hydrophobic polymeric dispersion agent dissolves or disperses; (B)further performing a secondary dispersion by adding a surfactant to theresultant dispersion liquid; and (C) performing a polymerization byusing a water-soluble radical polymerization initiator to obtain awater-absorbent resin as particles in a hydrous gel state which dispersein the petroleum hydrocarbon dispersion medium, wherein the hydrophilicpolymeric dispersion agent is at least one kind selected from the groupconsisting of polyvinyl pyrrolidone and polyvinyl alcohol.
 2. The methodfor producing a water-absorbent resin according to claim 1, furthercomprising: precipitating at least a part of the surfactant in a slurryof the dispersion medium containing the particles in a hydrous gel stateobtained in the step (C); and further performing two or three stages ofreversed-phase suspension polymerization by adding an aqueous solutionof the water-soluble ethylenically unsaturated monomer.
 3. The methodfor producing a water-absorbent resin according to claim 1, wherein thehydrophobic polymeric dispersion agent is at least one kind selectedfrom the group consisting of maleic anhydride-modified polyethylene,maleic anhydride-modified polypropylene, maleic anhydride-modifiedethylene-propylene copolymer, maleic anhydride-ethylene copolymer,maleic anhydride-propylene copolymer, maleicanhydride-ethylene-propylene copolymer, polyethylene, polypropylene,ethylene-propylene copolymer, oxidized polyethylene, oxidizedpolypropylene, and an oxidized ethylene-propylene copolymer.
 4. Themethod for producing a water-absorbent resin according to claim 1,wherein the surfactant is at least one kind selected from the groupconsisting of polyglyceryl fatty acid ester, sucrose fatty acid ester,and sorbitan fatty acid ester.
 5. The method for producing awater-absorbent resin according to claim 1, wherein the water-solubleethylenically unsaturated monomer is at least one kind selected from thegroup consisting of acrylic acid and its salt, methacrylic acid and itssalt, and acrylamide.
 6. The method according to claim 1, wherein thepetroleum hydrocarbon dispersion medium is at least one kind selectedfrom the group consisting of an aliphatic hydrocarbon and an alicyclichydrocarbon, having a carbon number of 6 to
 8. 7. A water-absorbentresin obtained by the method according to claim
 1. 8. The method forproducing a water-absorbent resin according to claim 2, wherein thehydrophobic polymeric dispersion agent is at least one kind selectedfrom the group consisting of maleic anhydride-modified polyethylene,maleic anhydride-modified polypropylene, maleic anhydride-modifiedethylene-propylene copolymer, maleic anhydride-propylene copolymer,maleic anhydride-ethylene-propylene copolymer, polyethylene,polypropylene, ethylene-propylene copolymer, oxidized polyethylene,oxidized polypropylene, and an oxidized ethylene-propylene copolymer. 9.The method for producing a water-absorbent resin according to claim 2,wherein the surfactant is at least one kind selected from the groupconsisting of polyglyceryl fatty acid ester, sucrose fatty acid ester,and sorbitan fatty acid ester.
 10. The method for producing awater-absorbent resin according to claim 3, wherein the surfactant is atleast one kind selected from the group consisting of polyglyceryl fattyacid ester, sucrose fatty acid ester, and sorbitan fatty acid ester. 11.The method for producing a water-absorbent resin according to claim 2,wherein the water-soluble ethylenically unsaturated monomer is at leastone kind selected from the group consisting of acrylic acid and itssalt, methacrylic acid and its salt, and acrylamide.
 12. The method forproducing a water-absorbent resin according to claim 3, wherein thewater-soluble ethylenically unsaturated monomer is at least one kindselected from the group consisting of acrylic acid and its salt,methacrylic acid and its salt, and acrylamide.
 13. The method forproducing a water-absorbent resin according to claim 4, wherein thewater-soluble ethylenically unsaturated monomer is at least one kindselected from the group consisting of acrylic acid and its salt,methacrylic acid and its salt, and acrylamide.